Longitudinal-effect type laminar piezoelectric/electrostrictive driver, and printing actuator using the driver

ABSTRACT

A laminar longitudinal-effect type piezoelectric or electrostrictive driver which is displaced upon application of an electric field thereto. The driver includes laminar electro-mechanical converting elements each undergoes a displacement in a direction of the electric field due to the longitudinal mode of the reverse piezoelectric effect or the electrostrictive effect, and the temperature compensating elements each of which is interposed between the appropriate adjacent two laminar electro-mechanical converting elements. The temperature compensating elements have a higher coefficient of linear thermal expansion in the direction of the electric field or direction of displacement of the driver. The driver is suitably usable for an actuator for operating a print element of a printer, for example.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a laminated piezoelectric/electrostrictive driver which provides a displacement due to a strain caused by a longitudinal mode effect of a piezoelectric or electrostrictive ceramic material, and also relates to a printing actuator using such a piezoelectric or electrostrictive driver for driving a printing element such as a print wire for a dot-matrix printing, for example.

2. Discussion of the Prior Art

A lamination type piezoelectric or electrostrictive element utilizing the longitudinal mode of the reverse piezoelectric effect or the electrostrictive effect. This laminar longitudinal-effect type piezoelectric or electrostrictive element consists of a plurality of piezoelectric ceramic films of lead zirconate titanate (PZT), for example, and a plurality of electrode films, which are alternately superposed on each other, in the direction in which the element is displaced (contracted and expanded) upon application of a voltage to the electrode films. The piezoelectric or electrostrictive element which is thus displaced is used as drivers for a dot-matrix impact print head, as an actuator for a VTR head, and an oscillator for a ultrasonic motor or other devices. In particular, there has been a growing attention to an actuator laminated in a process used for a laminar type ceramic capacitor, for its relatively high production efficiency and operating reliability.

An example of a piezoelectric printing actuator is generally shown at 100 in FIG. 5. This printing actuator 100 employs a generally elongate laminar longitudinal-effect type piezoelectric driver 101 whose linear displacement upon application of a voltage is utilized to drive a print element in the form of a print wire 11. The displacement is produced due to the longitudinal mode of the reverse piezoelectric effect or the electrostrictive effect, which is known in the art. The "longitudinal mode effect" means the reverse piezoelectric effect or the electrostrictive effect parallel to the direction of the electric field produced by the voltage applied to the driver, and is distinguished from the "transverse mode effect" perpendicular to the direction of the electric field. In the example of FIG. 5, the displacement occurs in the longitudinal direction of the generally elongate driver 101 in which piezoelectric ceramic films 42 are superposed, as shown in FIG. 6.

As shown in FIG. 5, the printing actuator 100 has a support holder having a main frame 2 provided with a base support block 3 at one end thereof. The piezoelectric driver 101 is supported by and accommodated in the support holder such that the driver 101 rests on the support block 3 at one of its longitudinal ends. A movable member 5 is fixed to the other end of the driver 101. The support holder also supports a link mechanism 16 disposed so as to guide the movable member 5 in the longitudinal direction of the driver 101 when the driver is displaced in the longitudinal direction. The movable member 5 is linked with a mechanism for amplifying the linear movement of the driver 101 into a larger longitudinal movement of the print wire 11. This converting mechanism includes a pair of sheet springs 6, 7, a tiltable member 8, and an arm 10. The sheet spring 6 is fixed at one end thereof to the main frame 2, while the sheet spring 7 is fixed at one end thereof to the movable member 5. The other ends of the sheet springs 6, 7 are fixed to the tiltable member 8, which is secured to one end of the arm 10. The print wire 11 is fixed at one end thereof to the other end of the arm 10. The linear displacement of the driver 101 produced upon application of a voltage thereto causes the sheet spring 7 to be displaced, thereby causing the tiltable member to be tilted. The tilting movement of the tiltable member 8 are transmitted to the print wire 11 through the arm 10, whereby the print wire 11 effects a printing movement toward a recording medium.

An example of the known laminar piezoelectric driver 101 is illustrated in FIG. 6. The driver 101 has 180 piezoelectric ceramic films 42 and 181 electrode films 41 which are alternately superposed on each other. Each piezoelectric ceramic film has a thickness of 98 μm, and each electrode film 41 has a thickness of 2 μm, so that the laminar piezoelectric driver 101 has a total thickness of about 18 mm. The piezoelectric ceramic film 42 has a piezoelectric constant d₃₃ of 6.35×10⁻¹⁰ m/V, and an electrostrictive constant of M₃₃ of 1.32×10⁻¹⁶ m/V². For the piezoelectric driver 101 to provide a linear displacement of 15 μm necessary to give a required movement to the print wire 11, 107V should be applied to the electrode films 41 to polarize the piezoelectric films 42 as indicated by arrows 45 in FIG. 6. Namely, the laminar piezoelectric driver 101 undergoes a linear displacement in its longitudinal direction (in the direction of lamination of the films 41, 42), due to the longitudinal mode of the reverse piezoelectric effect or electrostrictive effect parallel to the direction of the polarization.

The piezoelectric driver 101 has a property that the amount of residual strain remaining after the polarization of the piezoelectric ceramic films 42 is reduced as the temperature of the films 42 is elevated during use of the driver 101. Due to this phenomenon, the driver 101 has a very low or negative coefficient of linear thermal expansion (e.g., -3.8 ppm/°C.) at the operating temperature, unlike the other components of the printing actuator 100. Therefore, there exists a difference in the linear thermal expansion coefficient between the driver 101 and the main frame 2, which should be compensated for to assure reliable operation of the printing actuator 100. To this end, the main frame 2 is made of a metallic material having a low coefficient of linear thermal expansion, for example, an Invar alloy whose linear thermal expansion coefficient is +1.2 ppm/°C., and rigid temperature compensating members 12, 13 having a high positive linear thermal expansion coefficient are used in contact with the lower end of the driver 101. For instance, the temperature compensating members 12, 13 are aluminum blocks having a total thickness of 4 mm and a linear thermal expansion coefficient of +23.9 ppm/°C.

However, the material having a low linear thermal expansion coefficient for the frame 2 is considerably expensive, leading to an increased cost of the printing actuator 101. Further, it is cumbersome and time-consuming to position the temperature compensating members 12, 13 relative to the support block 3 and the piezoelectric driver 101. Thus, the use of the temperature compensating members 12, 13 pushes up the cost of manufacture. Further, there is a risk that the temperature compensating members 12, 13 are separated from the piezoelectric driver 101 during use, due to low bonding strength therebetween.

Where the piezoelectric driver 101 does not have a sufficiently high thermal conductivity, there may arise a temperature difference between the driver 101 and the temperature compensating members 12, 13. This temperature difference reduces the function of the temperature compensating members, and may cause an undesirable phenomenon that the operating end of the print wire 11 is advanced toward the recording medium as the operating temperature of the driver 101 increases. This results in the the tip of the print wire 11 contacting an ink ribbon or the recording medium while the print wire 11 is in the non-operated position.

The known piezoelectric driver 101 also suffers from a relatively high voltage (107V in the example of FIG. 6) required to obtain the desired printing movement of the print wire 11. One method of lowering the required voltage is to reduce the thickness of each piezoelectric ceramic film 42 and increase the number of the piezoelectric ceramic films. However, this solution suffers from another problem in sintering the driver 101. That is, there exists a difference in the sintering temperature of the driver 101, between the piezoelectric films 42 relatively near the longitudinal ends of the driver 101 and the films 42 in the middle of the lamination. This tends to lower the uniformity in the physical properties and the operating reliability of the driver 101.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide a laminar longitudinal-effect type piezoelectric/electrostrictive driver which is reliable in operation, with a reduced influence by a variation in the operating temperature.

The above object may be achieved according to the principle of the present invention, which provides a laminar longitudinal-effect type piezoelectric/electrostrictive driver which is displaced upon application of an electric field thereto, the driver comprising a plurality of laminar electro-mechanical converting elements and a plurality of temperature compensating elements which are alternately superposed on each other. Each laminar electro-mechanical converting element undergoes a displacement in a direction of the electric field, due to a longitudinal piezoelectric/electrostrictive effect, and has a first coefficient of linear thermal expansion in the direction of the electric field. The temperature compensating elements have a second coefficient of linear thermal expansion in the direction of the electric field, which is higher than the first coefficient of linear thermal expansion of the laminar electro-mechanical converting elements.

In the piezoelectric/electrostrictive driver of the present invention constructed as described above, the difference between the operating temperatures of the piezoelectric/electrostrictive electro-mechanical converting elements and the temperature compensating elements is relatively low because of the alternate superposition or lamination of these two different elements having the respective different linear thermal expansion coefficients. This arrangement assures a reduced deviation of the displacement amount of the driver from the nominal value. Further, the present driver does not require a conventionally used separate temperature compensating member, which should be accurately positioned relative to the driver and whose operating temperature is considerably different from that of the driver. Where the driver is used for operating a print element of a printer, the print element can be moved between predetermined constant operated and non-operated positions, irrespective of the operating temperature of the driver.

Generally, each laminar electro-mechanical converting element which exhibits a longitudinal mode of the piezoelectric or electrostrictive effect consists of a plurality of piezoelectric ceramic films and a plurality of electrode films which are alternately superposed on each other such that each piezoelectric ceramic film is sandwiched between the adjacent two electrode films.

It is a second object of the invention is to provide such a piezoelectric/electrostrictive driver which is economical to manufacture and is highly durable in operation.

The above object may be achieved according to one preferred form of the present invention, wherein the laminar piezoelectric/electrostrictive electro-mechanical converting elements and the temperature compensating elements form a co-sintered laminar structure. Namely, the driver is produced by co-firing the unfired alternately superposed layers for the electro-mechanical converting and temperature compensating elements. This co-fired or co-sintered laminar structure is free from separation or flake-off of the electro-mechanical elements and the temperature compensating elements from each other and has a high bonding strength between the elements and a comparatively high degree of operating durability. That is, it is not necessary to bond a separate temperature compensating member to the driver by an adhesive, while maintaining an accurate relative position therebetween, which is a difficult and cumbersome procedure and pushes up the cost of manufacture of an actuator device using the driver.

Where the temperature compensating elements are co-sintered with the piezoelectric/electrostrictive electro-mechanical converting elements, each temperature compensating element is preferably a laminar structure consisting of a plurality of temperature compensating films which are superposed on each other between the two adjacent laminar electro-mechanical converting elements. The temperature compensating elements or films are preferably formed of a piezoelectric ceramic material (such as a ceramic material whose major component is lead zirconate titanate) whose composition is substantially the same as that of the piezoelectric/electrostrictive converting elements.

It is a third object of the present invention to provide a laminar longitudinal-effect type piezoelectric/electrostrictive driver which has piezoelectric ceramic films each having a relatively small thickness and which requires a relatively small electric energy to provide a desired displacement.

The above object may be achieved according to another form of the present invention, wherein each of the plurality of laminar electro-mechanical converting elements consists of a plurality (or a multiplicity) of piezoelectric ceramic films and a plurality (or a multiplicity) of electrode films which are alternately superposed on each other, and wherein each of the plurality of temperature compensating elements is a single temperature compensating sheet bonded by an adhesive to the adjacent laminar electro-mechanical converting elements. In the present form of the invention, the thickness of each piezoelectric ceramic film of each laminar electro-mechanical converting element can be made relatively small (e.g., 40 μm), since the total number of the piezoelectric ceramic films can be relatively large by increasing the number of each laminar electro-mechanical converting element, and/or the number of the piezoelectric films of each electro-mechanical converting element. Consequently, the required amount of electric energy to produce electric field for obtaining a desired amount of displacement of the driver can be accordingly reduced.

The driver constructed according to the above form of the invention may be prepared by sintering the individual laminar electro-mechanical elements, and bonding these sintered laminar elements together with a temperature compensating sheet interposed between the adjacent sintered laminar elements. In sintering each laminar electro-mechanical element, there exists a relatively small difference in the sintering temperature between the piezoelectric ceramic films at different positions. Thus, the present driver has uniform physical properties and piezoelectric/electrostrictive characteristics.

Further, the above arrangement permits the temperature compensating sheets to have a relatively large thickness, since the desired amount of displacement of the driver can be obtained applying a relatively small electric energy to the relatively thin piezoelectric ceramic films. Accordingly, the overall coefficient of linear thermal coefficient of the driver can be made relatively high, thereby permitting the driver to be supported by a support frame which has a relatively high coefficient of linear thermal expansion and which is accordingly inexpensive.

It is a fourth object of the present invention to provide a printing actuator for operating a print element of a printer, which actuator uses such a laminar longitudinal-effect type piezoelectric/electrostrictive driver, for reduced cost of manufacture and increased operating reliability of the printer.

The above fourth object may be accomplished according to another aspect of the present invention, which provides a printing actuator for operating a print element of a printer, comprising a laminar longitudinal-effect type piezoelectric/electrostrictive driver which is displaced upon application of an electric field thereto, a support frame having a support member for supporting the driver at one of opposite ends of the driver as viewed in a direction of displacement of the driver, and a mechanism for transmitting a displacement of the driver to the print element. The laminar longitudinal-effect type piezoelectric/electrostrictive driver including a plurality of laminar electro-mechanical converting elements each of which undergoes the displacement in the direction parallel to the direction of the electric field applied thereto, due to a longitudinal piezoelectric/electrostrictive effect. The support frame has a first coefficient of linear thermal expansion in the direction of displacement of the driver, while the laminar electro-mechanical converting elements have a second coefficient of linear thermal expansion in the same direction, which second coefficient is smaller than the first coefficient of the support frame. The driver further includes a plurality of temperature compensating elements each of which is interposed between the adjacent two laminar electro-mechanical converting elements such that the temperature compensating elements and the laminar electro-mechanical converting elements are alternately superposed on each other. The temperature compensating elements have a third coefficient of linear thermal expansion in the direction of displacement of the driver, which third coefficient is higher than the first coefficient of the support frame, so that an overall coefficient of linear thermal expansion of the laminar piezoelectric/electrostrictive driver is substantially equal to the first coefficient of the support frame.

The printing actuator of the present invention constructed as described above has the advantages described above with respect to the piezoelectric/electrostrictive driver constructed so as to achieve the first object of the present invention. Further, the present printing actuator provides a high degree of freedom in selecting the material for the support frame, since the overall coefficient of linear thermal expansion of the driver which should be almost equal to that of the support frame can be adjusted by changing the coefficient of linear thermal expansion of the temperature compensating elements. Namely, the support frame having a relatively high linear thermal expansion coefficient can be used if the temperature compensating elements have a relatively high linear thermal expansion coefficient. In this case, the support frame is accordingly inexpensive.

For instance, each temperature compensating member may be a relatively thick temperature compensating sheet such as an aluminum sheet, which has a relatively high linear thermal expansion coefficient. In this case, the support frame may be formed of a sintered steel material. For suitable sintering of the driver, it is desirable that the individual laminar electro-mechanical converting elements are preferably sintered, without the temperature compensating sheets, so that the sintered electro-mechanical converting elements are bonded together with each temperature compensating sheets interposed between the adjacent electro-mechanical converting elements.

However, the laminar electro-mechanical converting elements and the temperature compensating elements may be a co-fired or co-sintered laminar structure. In this case, the cost of manufacture of the driver is reduced, and the durability of the driver is increased. In this case, each temperature compensating element is preferably a laminar structure consisting of two or more temperature compensating films superposed on each other between the adjacent two laminar electro-mechanical converting elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and optional objects, features and advantages of the present invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a printing actuator constructed according to one embodiment of this invention;

FIG. 2 is a perspective view of a piezoelectric driver used in the printing actuator of FIG. 1;

FIG. 3 is a side elevational view of a printing actuator using a modified piezoelectric driver constructed according to another embodiment of the present invention;

FIG. 4 is a perspective view of the piezoelectric driver used in the actuator of FIG. 3;

FIG. 5 is a side elevational view of a conventional printing actuator using a known piezoelectric driver; and

FIG. 6 is a perspective view of the known driver used in the actuator of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 and 2, reference numeral 50 denotes a printing actuator for operating a print element in the form of a print wire 11 of a dot-matrix impact printer. Usually, the printer has two or more print wires, and each print wire is operated by the printing actuator 50. For easy understanding of the present embodiment as distinguished from the known printing actuator 100 of FIGS. 5 and 6 described above, the same reference numerals as used in FIGS. 5 and 6 are used in FIGS. 1 and 2, to identify the corresponding components or parts which are substantially identical with those of the actuator 100.

The printing actuator 50 uses a support frame A for supporting a laminar longitudinal-effect type piezoelectric/electrostrictive driver 50 (hereinafter simply referred to as "piezoelectric driver"). The support frame A is formed of a sintered steel material having a coefficient of linear thermal expansion of +12.1 ppm/°C. The support frame A includes a main frame member 2, a base support block 3 and a secondary frame member 4, which are arranged relative to each other to define a generally U-shaped structure as seen in FIG. 1. The main and secondary frame members 2, 4 are disposed parallel to each other, and are spaced apart from each other by the support block 3, which extends from a lower part (in FIG. 1) of the main frame member 2. The piezoelectric driver 1 is a generally elongate member, and is accommodated in the support frame A such that the driver 1 rests at its lower end on the support block 3. The lower end face of the driver 1 is boned to the to face of the support block 3. As described below, the piezoelectric driver 1 is displaced in the longitudinal direction parallel to the direction of extension of the parallel frame members 2, 4, upon application of a voltage to the driver 1.

A movable member 5 is fixed by bonding to the upper end (in FIG. 1) of the piezoelectric driver 1, such that the movable member 5 is disposed opposite to an upper part of the main frame member 2. Between the opposed surfaces of the movable member 5 and the main frame member 2, there are disposed a pair of parallel sheet springs 6, 7 such that the sheet spring 6 is secured at one end thereof to the main frame member 2 while the sheet spring 7 is fixed at one end thereof to the movable member 5. The two sheet springs 6, 7 are spaced apart from each other by a suitable distance in the direction perpendicular to the direction of movement of the movable member 5, and are fixed at the other ends to a tiltable member 8 which is moved with a pivotable arm 10. The tiltable member 8 is fixed to one end of the arm 10, and the print wire 11 is fixed at one end to the other other end of the arm 10.

At an upper part (in FIG. 1) of the secondary frame member 4, there is supported a quadric parallel link mechanism 16 formed from sheet springs. The link mechanism 16 is associated with the movable member 5 so that the link mechanism 16 functions to guide the movable member 5 in the longitudinal direction of the driver 1, when the driver 1 is displaced (expanded and contracted).

In operation of the printing actuator 50, the sheet spring 7 is displaced parallel to the other sheet spring 6 when the movable member 5 is moved as the piezoelectric driver 1 is displaced with an electric field applied thereto, whereby the tiltable member 8 is tilted to pivot the arm 10 about the tiltable member 8, to thereby displace the print wire 11 in the longitudinal direction.

More specifically described, the piezoelectric driver 1 is expanded toward the movable member 5 upon application of a voltage thereto, whereby the movable member 5 is moved upward (in FIG. 1) so as to flex the sheet springs 6, 7 for rotating the tiltable member 8 in the counterclockwise direction (as seen in FIG. 1). As a result, the print wire 11 is advanced toward a recording medium for printing a dot on the medium.

When the applied voltage is removed from the piezoelectric driver 1, the driver is restored to its original position, whereby the movable member 5 and sheet springs 6, 7 are also returned to the original positions, with a clockwise movement of the tiltable member 8. Thus, the print wire 11 is retracted to the non-operated position, which is established by abutting contact of the arm 10 with a stop 35 fixed to the support frame A.

When the piezoelectric driver 1 is reciprocated in the longitudinal direction as described above, the link mechanism 16 is elastically deformed for guiding the movable member 5 in the longitudinal direction of the driver 1.

The piezoelectric driver 1 consists of five laminar piezoelectric driver elements 40 as laminar electro-mechanical converting elements, and four temperature compensating sheets 43 as temperature compensating elements, as shown in FIG. 2. Each laminar piezoelectric driver element 40 has 46 electrode films 41 and 45 piezoelectric ceramic films 42, which are alternately superposed on each other and co-sintered such that each piezoelectric ceramic film 42 is sandwiched between the adjacent two electrode films 41. In the interest of simplification and brevity, only three piezoelectric ceramic films 42 are shown in FIG. 2, for each piezoelectric driver element 40. Each piezoelectric ceramic film 42 has a thickness of 40 μm, and a piezoelectric constant d₃₃ of 6.35×10 ⁻¹⁰ m/V and an electrostrictive constant M₃₃ of 1.32×10 ⁻¹⁶ m/V². Each electrode film 41 has a thickness of 2 μm. Each laminar piezoelectric driver element 40 has a total thickness of 1.89 mm, and undergoes a linear displacement in the direction of lamination due to the longitudinal mode of the reverse piezoelectric effect or electrostrictive effect.

Each of the four temperature compensating sheets 43 is formed of aluminum and has a thickness of 3.15 mm. The five sintered laminar piezoelectric driver elements 40 and the four temperature compensating films 43 are alternately superposed on each other and bonded together by a known adhesive into the laminar longitudinal-effect type piezoelectric driver 1, which has a length of 22 mm along which the driver 1 is displaced upon application and removal of a voltage to and from the electrode films 41.

Due to reduction in the amount of residual strain of the polarized piezoelectric ceramic films 42 at an elevated operating temperature of the driver 1, the laminar piezoelectric driver elements 40 have a coefficient of linear thermal expansion of -3.8 ppm/°C. On the other hand, the aluminum temperature compensating sheets 43 have as high as +23.9 ppm/°C. Therefore, the overall linear thermal expansion coefficient of the piezoelectric driver 1 is +12.1 ppm/°C., which is substantially the same as that of the support frame A.

In the present printing actuator 50, a displacement of 15 μm of the piezoelectric driver 1 necessary for a printing movement of the print wire 11 was obtained by applying 76V between each pair of electrode films 41 on the opposite surfaces of each piezoelectric ceramic film 42 of the driver elements 40. This required voltage value of 76V is considerably reduced as compared with the required voltage of 107V in the known actuator 100 of FIGS. 5 and 6. The piezoelectric driver 1 is also advantageous for the relatively small thickness (40 μm) of the piezoelectric ceramic films 42 and the relatively small number (45) of these ceramic films 42, which result in the comparatively small total thickness of each laminar piezoelectric driver element 40. This reduced thickness allows easy fabrication of the piezoelectric driver 1 with uniform grain size and density of the laminar piezoelectric driver elements 40, and assures improved piezoelectric and insulating properties of the driver elements 40. In this respect, the piezoelectric driver 1 is less likely to be defective and is more reliable and stable in operation than the known counterpart 101.

Since the laminar piezoelectric driver 1 uses the relatively thin laminar piezoelectric driver elements 40 as electro-mechanical converting elements capable of providing a desired amount of displacement or strain with a relatively small electric energy, the ratio of the total thickness of the electro-mechanical converting elements 40 to that of the length of the driver 1 can be made relatively small. In other words, the ratio of the total thickness of the temperature compensating sheets 43 to the length of the driver 1 can be easily increased while assuring the desired amount of displacement of the driver 1. Consequently, the overall coefficient of linear thermal expansion of the driver 1 can be made as high (+12.1 ppm/°C.) as that of the sintered steel material of the support frame A. Thus, the present piezoelectric driver 1 makes it possible to use a comparatively inexpensive material for the frame A. Further, the overall linear thermal expansion coefficient of the driver 1 can be adjusted as needed by changing the ratio of the total thickness of the temperature compensating sheets 43 to the length of the driver 1. This adjustment permits the use of various materials for the frame A.

It is also noted that the present printing actuator 50 does not use a separate temperature compensating member like the member 12 used in the known actuator 100 of FIGS. 5 and 6. Since the temperature compensating sheets 43 are provided as integral parts of the piezoelectric driver 1, the difference in the temperature between the piezoelectric driver elements 40 and the temperature compensating sheets 43 is extremely smaller than the difference between the piezoelectric driver 101 and the temperature compensating member 12. According to an experiment, the temperature difference in the piezoelectric driver 1 of the present actuator 50 is as small as about 2° C. while that in the known piezoelectric driver 101 is as large as 25° C. Accordingly, the temperature compensating sheets 43 effectively function to minimize a variation in the amount of linear displacement of the driver 1 due to a change in the operating temperature, and thereby protect the print wire 11 from an undesirable contact with a print ribbon or recording medium while the print wire 11 is in the non-operated position.

The piezoelectric driver 1 may be suitably modified depending upon the specific requirements of the printing actuator 50. For example, the following modifications are possible to provide the desired amount of linear displacement of the driver 1, with a further reduced electric energy, if the support frame A is formed of a material having a relatively low coefficient of linear thermal expansion:

According to one modification, each laminar piezoelectric driver element 40 uses 86 piezoelectric ceramic films 42 (each having the 40 μm thickness) each sandwiched by the adjacent electrode films 41 (having the 2 μm thickness), so that the driver element 40 has a thickness of 3.61 mm. On the other hand, each aluminum temperature compensating sheet 43 has a thickness of 1 mm. In this case, too, the four temperature compensating sheets 43 and the five piezoelectric driver elements 40 are superposed on each other and bonded together by an adhesive to provide the piezoelectric driver 1 whose length is 22 mm. This arrangement makes it possible to reduce the required voltage of the driver 1 to 45V.

According to another modification, each laminar piezoelectric driver element 40 uses 164 piezoelectric ceramic films 42 each of which has a thickness of 20 μm and is sandwiched by the adjacent electrode films 41 having a thickness of 2 μm, so that the thickness of each driver element 40 is 3.61 mm. In this case, too, the four aluminum temperature compensating sheets 43 each having a 1 mm thickness are used with the five piezoelectric driver elements 40, so that the length of the obtained piezoelectric driver 1 is 22 mm. In this arrangement, the required voltage to energize the driver 1 is lowered down to 23V.

Referring next to FIGS. 3 and 4, there will be described another embodiment of the present invention. In FIG. 3, reference numeral 60 denotes a printing actuator using a laminar longitudinal-effect type piezoelectric/electrostrictive driver 21 (hereinafter referred to simply as "piezoelectric driver"), which is different from the piezoelectric driver 1 of the actuator 50 of FIGS. 1 and 2.

The printing actuator 60 uses a U-shaped support frame B which is formed of an Invar alloy having a linear thermal expansion coefficient of 2.0 ppm/°C. at a temperature between the room temperature and 100° C. Unlike the piezoelectric driver 1 of the preceding embodiment, the piezoelectric driver 21 rests at its lower end on a pre-load member 22 provided in the support frame B. The pre-load member 22 has tapered surfaces for moving the driver 21 in the longitudinal direction, so as to adjust a pre-load with respect to the movable member 5 fixed to the sheet spring 7. The movable member 5 is bonded to the upper end face of the driver 21. A linear displacement of the driver 21 is converted into a printing movement of the print wire 11, in substantially the same manner as in the preceding embodiment.

It is desirable that the laminar piezoelectric driver 21 has substantially the same linear thermal expansion coefficient as the support frame B. To this end, the driver 21 is constructed as described below.

As shown in FIG. 4, the laminar piezoelectric driver 21 consists of five laminar piezoelectric driver elements 40 as electro-mechanical converting elements, and six laminar temperature compensating layers 44 as temperature compensating elements. These driver elements 40 and temperature compensating layers 44 are alternately superposed on each other and bonded together. Each piezoelectric driver element 40 consists of silver palladium electrode films 41 each having a thickness of 4 μm, and lead zirconate titanate (PZT) piezoelectric ceramic films 42 each having a thickness of 100 μm. These electrode and ceramic films 41, 42 are alternately superposed on each other and co-fired into the drive element 40 which has a total thickness of 2 mm.

Each laminar temperature compensating layer 44 consists of 13 temperature compensating films 46 (only two films 46 shown in FIG. 4) each having a 100 μm thickness that the laminar layer 44 has a total thickness of 1.3 mm. The temperature compensating layers 44 are formed of a ceramic material containing zirconia and suitable additives, so that the sintering temperature and shrinkage ratio of the layers 44 are close to those of the ceramic material (PZT) of the piezoelectric ceramic films 42. The electrode films 41, piezoelectric ceramic layers 42 and temperature compensating films 46 are formed by a suitable lamination technique such as doctor blade method and thick-film printing, subjected to a process to remove the binder, and co-sintered at 1100°-1300° C. into the laminar longitudinal-effect type piezoelectric driver 21 which has 3 mm (depth)×3 mm (width)×17.8 mm (height or length) dimensions. The thus prepared piezoelectric driver 21 is subjected to polarization treatment for 10 minutes with 3 kV/mm, at 150° C., to give the driver 21 the required piezoelectric or electrostrictive properties.

The laminar piezoelectric driver elements 40 and the laminar temperature compensating layers 44 have linear thermal expansion coefficients of -4.0 ppm/°C. and 10.0 ppm/°C., respectively, at a temperature between the room temperature and 100° C. The overall linear thermal expansion coefficient of the piezoelectric driver 21 is calculated as 2.1 ppm/°C.={-4.0 ppm/°C.×(2 mm×5)+10.0 ppm/°C.×(1.3 mm×6)}/17.8 mm. This coefficient 2.1 ppm/°C. of the driver 21 is considerably larger than -4.0 ppm/°C., the coefficient of the driver elements 40 which provide a displacement due to the longitudinal mode of the reverse piezoelectric effect or restrictive effect. Accordingly, there exists a relatively small difference in the thermal expansion coefficient between the piezoelectric driver 21 and the support frame B.

In the present embodiment, the laminar piezoelectric driver elements 40 and the laminar temperature compensating layers 44 are co-sintered and thus bonded together, assuring improved bonding strength therebetween, and accordingly improved structural durability and reduced cost of manufacture of the driver 21. The elimination of a separate temperature compensating member contributes to further reduction in the cost of manufacture of the driver 21.

In the present piezoelectric driver 21, too, there is a temperature difference of only about 2° C. between the laminar piezoelectric driver elements 40 and the laminar temperature compensating layers 44.

The laminar temperature compensating layers 44 may be formed of a lead zirconate titanate (PZT) ceramic material having substantially the same composition as that of the piezoelectric ceramic layers 42. In this case, Each layer 44 consists of 20 temperature compensating films 46 each having a thickness of 100 μm, and has a total thickness of 2 mm. The piezoelectric driver 21 using the thus modified temperature compensating layers 44 has a length of 22 mm. In this modified embodiment, only the piezoelectric driver elements 40 are subjected to a polarization process for 10 minutes with 3 kV/mm at 150° .C, and the ceramic films 46 of the temperature compensating layers 44 remain unpolarized to avoid expansion and contraction due to the reverse piezoelectric effect or electrostrictive effect.

In the modified piezoelectric driver 21 described above, the piezoelectric driver elements 40 have a linear thermal expansion coefficient of -4.0 ppm/°C. (at a temperature between the room temperature and 100° C.) as in the above embodiment, but the linear thermal expansion coefficient of the laminar ceramic temperature compensating layers 44 is 7.0 ppm/°C., since the layers 44 remain unpolarized and the thickness of the layers 44 are not changed due to a variation in the residual strain with a change in the operating temperature. The overall coefficient of linear thermal expansion of the piezoelectric driver 21 is calculated as 2.0 ppm/°C.={-4.0 ppm/°C.×(2 mm×5)+7.0 ppm/°C.×(2 mm×6)}/22 mm, which is considerably larger than the linear thermal expansion coefficient of -4.0 ppm/°C. of the piezoelectric driver elements 40. Consequently, there exists only a small difference in the linear thermal expansion coefficient between the driver 21 and the support frame B.

Further, since the temperature compensating films 46 are formed of the same ceramic material as the piezoelectric ceramic films 42, the films 42, 46 may be easily formed as green sheets and co-sintered, resulting in significantly reduced cost of manufacture and improved bonding strength of the driver 21.

While the present invention have been described in its presently preferred embodiments in the form of the printing actuator 50, 60 incorporating the laminar longitudinal-effect type piezoelectric/electrostrictive driver 1, 21, it is to be understood that the present invention is not limited to the details of the illustrated embodiments, but may be embodied with various changes and modifications which may occur to those skilled in the art, in view of the foregoing teachings. For instance, the dimensions of the laminar piezoelectric driver elements 40, temperature compensating sheets 43 and laminar temperature compensating layers 44, and the materials of the electrode films 41, piezoelectric ceramic films 42 and temperature compensating sheets 43 or films 46 may be suitable modified or changed, depending upon the required linear thermal expansion coefficients relative to that of the support frame A, B, desired amount of displacement of the driver 1, 21, and desired electric energy applied to the driver to obtain the desired operating stroke of the print wire 11.

Although the piezoelectric/electrostrictive driver 1, 21 in the illustrated embodiments is used for the printing actuator 50, 60 for a dot-matrix impact printer, the driver constructed according to the present invention may be used for any other actuators as described in the introductory part of the present specification. 

What is claimed is:
 1. A laminar, longitudinal-effect type piezoelectric-electrostrictive driver which is displaced upon application of an electric field thereto, comprising:a plurality of laminar electro-mechanical converting elements each of which undergoes a displacement in a direction of said electric field, due to a longitudinal piezoelectric/electrostrictive effect, said laminar electro-mechanical converting elements having a first coefficient of linear thermal expansion in said direction; a plurality of temperature compensating elements having a second coefficient to linear thermal expansion in said direction, which is higher than said first coefficient, said laminar electro-mechanical converting elements and said temperature compensation elements being alternately superposed on each other, and each of said plurality of temperature compensating elements is formed of a metallic material.
 2. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 1, wherein each said laminar electro-mechanical converting element consists of a plurality of piezoelectric ceramic films and a plurality of electrode films which are alternately superposed on each other such that each of said piezoelectric ceramic films is sandwiched between the adjacent two electrode films.
 3. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 1, wherein each said temperature compensating element is a laminar structure consisting of a plurality of temperature compensating films superposed on each other between the adjacent two laminar electro-mechanical converting elements.
 4. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 1, wherein each of said temperature compensating elements consists of a single temperature compensating sheet bonded by an adhesive to the adjacent laminar electro-mechanical converting elements.
 5. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 4, wherein each said temperature compensating sheet consists of aluminum.
 6. A laminar longitudinal-effect type piezoelectric/electrostrictive driver which is displaced upon application of an electric field thereto, comprising:at least one laminar electro-mechanical converting element each of which undergoes a displacement in a direction of said electric field, due to a longitudinal piezeoelectric/electrostrictive effect, said laminar electro-mechanical converting elements having a first coefficient of linear thermal expansion in said direction, each said laminar electro-mechanical converting element comprising a plurality of piezoelectric ceramic films and a plurality of electrode films which are alternately superposed on each other such that each of said piezoelectric ceramic films is sandwiched between the adjacent two electrode films; and at least one laminar temperature compensating element having a second coefficient of linear thermal expansion in said direction, which is higher than said first coefficient, each said laminar temperature compensating element comprising a plurality of temperature compensating films superposed on each other, said at least one laminar electro-mechanical converting element and said at least one laminar temperature compensating element being alternately superposed on each other and cooperating to constitute a co-sintered structure.
 7. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 6, wherein said temperature compensating elements are formed of a piezoelectric ceramic material having substantially a same composition as said electro-mechanical converting elements.
 8. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 7, wherein said temperature compensating elements are formed of a ceramic material whose major component consists of lead zirconate titanate.
 9. A laminar longitudinal-effect type piezoelectric/electrostrictive driver according to claim 8, wherein said temperature compensating elements are unpolarized.
 10. A printing actuator for operating a print element of a printer, comprising:a laminar longitudinal-effect type piezoelectric/electrostrictive driver which is displaced upon application of an electric field thereto; a support frame having a support member for supporting said driver at one of opposite ends of said driver as viewed in a direction of displacement thereof, said support frame having a first coefficient of linear thermal expansion in said direction of displacement; and a mechanism for transmitting a displacement of said driver to said print element; said laminar longitudinal-effect type piezoelectric/electrostrictive driver including a plurality of laminar electro-mechanical converting elements each of which undergoes said displacement in said direction of displacement parallel to a direction of said electric field, due to a longitudinal piezoelectric/electrostrictive effect, said laminar electro-mechanical converting elements having a second coefficient of linear thermal expansion in said direction of displacement, said second coefficient being smaller than said first coefficient; said driver further including a plurality of temperature compensating elements, said laminar electro-mechanical converting elements and said temperature compensating elements being alternately superposed on each other, said temperature compensating elements having a third coefficient of linear thermal expansion in said direction, which is higher than said first coefficient of said support frame, so that an overall coefficient of linear thermal expansion of said laminar piezoelectric/electrostrictive driver is substantially equal to said first coefficient, each of said plurality of temperature compensating elements being formed of a metallic material.
 11. A printing actuator according to claim 10, wherein each of said temperature compensating elements consists of a single temperature compensating sheet bonded by an adhesive to the adjacent laminar electro-mechanical converting elements.
 12. A printing actuator according to claim 11, wherein each said temperature compensating sheet consists of aluminum.
 13. A printing actuator according to claim 11, wherein said support frame consists of a sintered steel material.
 14. A printing actuator for operating a print element of a printer, comprising:a laminar longitudinal-effect type piezoelectric/electrostrictive driver which is displaced upon application of an electric field thereto; a support frame having a support means for supporting said driver at one of opposite ends of said driver as viewed in a direction of displacement thereof, said support frame having a first coefficient of linear thermal expansion in said direction of displacement; and a mechanism for transmitting a displacement of said driver to said print element; said laminar longitudinal-effect type piezoelectric/electrostrictive driver including a plurality of laminar electro-mechanical converting elements each of which undergoes said displacement in said direction of displacement parallel to a direction of said electric field, due to a longitudinal piezoelectric/electrostrictive effect, said laminar electro-mechanical converting elements having a second coefficient of linear thermal expansion in said direction of displacement, said second coefficient being smaller than said first coefficient; each said laminar electro-mechanical converting element comprising a plurality of piezoelectric ceramic films and a plurality of electrode films which are alternately superposed on each other such that each of said piezoelectric ceramic films is sandwiches between the adjacent two electrode films; said driver further including a plurality of temperature compensating elements, said laminar electro-mechanical converting elements and said temperature compensating elements being alternately superposed on each other, said temperature compensating elements having a third coefficient of linear thermal expansion in said direction, which is higher than said first coefficient of said support frame, so that an overall coefficient of linear thermal expansion of said laminar piezoelectric/electrostrictive driver is substantially equal to said first coefficient, each said laminar temperature compensating element comprising a plurality of temperature compensating films superposed on each other, and at least one laminar electro-mechanical converting element and said at least one laminar temperature compensating element being alternately superposed on each other and cooperating to constitute a co-sintered structure.
 15. A printing actuator according to claim 14, wherein said laminar electro-mechanical converting elements and said temperature compensating elements constitute a co-fired laminar structure.
 16. A printing actuator according to claim 15, wherein each of said temperature compensating elements is a laminar structure consisting of a plurality of temperature compensating films superposed on each other between the adjacent two laminar electro-mechanical converting elements.
 17. A printing actuator according to claim 15, wherein said temperature compensating elements are formed of a piezoelectric ceramic material having substantially a same composition as said electro-mechanical converting elements. 